1. Field of the Invention
The present invention relates to semiconductor storage devices, for example, relates to a sense circuit used in a NAND type flash memory.
2. Related Art
In recent years, there has been expanding interest in the application of treating high capacity data such as images and moving pictures by mobile equipment. With the expansion, demands for semiconductor storage devices such as a NAND type flash memory used in mobile equipment have been rapidly increasing. Since the NAND type memory adopts a NAND string structure to which memory cells are connected in series, it is excellent in miniaturization; however, a cell current flowing through the memory cells is relatively small. Therefore, a sense amplifier of the NAND type flash memory needs to accurately detect a small cell current.
The sense amplifier is provided with a capacitor which is connected to a sense node and a precharge circuit which charges a bit line. The precharge circuit serves to preliminarily charge the bit line and the capacitor before data detection, and to hold a bit line potential.
For example, when a selected memory cell connected to the bit line allows a relatively large current (cell current) into the selected memory cell, that is, when the selected memory cell is a memory cell in ON state (hereinafter, also referred to as “ON cell”); first, the capacitor connected to the sense node supplies a charge to the bit line during the data detection. When the charge accumulated in the capacitor reduces, a potential at the sense node reduces. When the potential at the sense node is lowered to a predetermined potential, a precharge circuit supplies the charge to the bit line in place of the capacitor. At this time, if the potential at the sense node does not sufficiently reduce, the sense amplifier is not normally operated; and consequently, there is a case where the memory cell cannot recognize the ON cell.
The precharge circuit is connected to bit line via a plurality of transistors. The potential at the sense node during supplying the charge by the precharge circuit is determined in response to a gate voltage of their transistors. Therefore, in order to lower the potential at the sense node, it is conceivable to lower the gate voltage of the transistors provided between the precharge circuit and the bit line. However, a bit line potential during the data detection is also determined by the gate voltage of these transistors. Consequently, to lower the gate voltage of these transistors makes the cell current lower by lowering of the bit line potential. This lowering of the cell current leads to an increase in data detection time.